Differences in the distribution and seed germination behaviour of populations of Bromus rigidus and Bromus diandrus in South Australia: adaptations to habitat and implications for weed management

A field survey was undertaken in South Australia to determine the relative distribution of the brome grass species B. diandrus and B. rigidus. Seeds of brome grass plants were collected from locations across the Yorke (n = 10) and Eyre Peninsulas (n = 25). B. rigidus was found more frequently and at higher densities in South Australian crops than B. diandrus, which showed a distinct preference for undisturbed fence-line margins. Species identity of brome plants in each sample was initially determined by assessing morphology of the callus-scar of the caryopsis as well as the structure of the panicle. Species identity was later confirmed by counting somatic chromosome number. There was consistent agreement between the 2 approaches to identification, indicating that these morphological features can be used with confidence when identifying B. diandrus and B. rigidus in the field. Although B. diandrus and B. rigidus are morphologically very similar, they showed large differences in germination behaviour. B. diandrus seeds collected from fence-line margins were more germinable than B. rigidus from neighbouring cropped areas. Populations of B. rigidus also showed strong inhibition of seed germination when exposed to light. This inhibitory effect of light on seed germination was not seen in the B. diandrus collections. Two populations of B. rigidus from Yorke Peninsula showed little germination (<15% germination in complete darkness) until well after the start of the next growing season. These 2 populations did, however, show a large response to treatment with gibberellic acid (1 mm), indicating high seed viability but presence of deep dormancy. From a practical point of view, the germination behaviour (longer dormancy and light inhibition) exhibited by B. rigidus would allow this species to proliferate under conservation tillage systems such as no-till, where seeds only experience complete darkness after burial following the sowing operation. Germination behaviour of B. rigidus observed in this study is expected to contribute to greater seed carry-over from one season to the next, and favour its colonisation in crops, as seen in the current field survey.